1. Technical Field of the Invention
The present invention relates generally to a physical parameter measuring system which applies oscillations to a blood vessel in a person or subject and samples and analyzes the oscillations transmitted through the blood vessel to obtain living body information noninvasively, and more particularly to an improvement on a physical parameter measuring system designed to obtain living body information in the form of a parameter derived based on the center of an arc defined by mapping on a two-dimensional plane samples of the transmitted oscillations moving backward and forward in time sequence.
2. Background Art
As typical noninvasive systems for measuring the blood pressure, an oscillometric system a Korotkoff system are known. The oscillometric system is designed to exert pressure on a patient""s upper arm using a cuff and changing the pressure to monitor a change in amplitude of a resulting pulsation corresponding to the blood pressure. The Korotkoff system is designed to detect Korotkoff sounds produced from a blood vessel on which pressure is exerted by a cuff to determine the blood pressure. These systems, however, require approximately thirty seconds for one measurement and are useless in an emergency case where it is necessary to monitor a sudden change in blood pressure of a patient on the operating table.
U.S. Pat. No. 5,590,649 issued on Jan. 7, 1997 discloses a blood pressure determining system designed to avoid the above problem. The system detects the elasticity of a blood vessel which will change as a change in blood pressure to determine the blood pressure and will be described below in detail with reference to FIGS. 8 and 9.
The system includes generally an oscillator 1, an exciter 2, a sensor 3, a phase detector 4, and A/D converters 5. The oscillator 1 generates a since wave of several tens Hz to oscillate an artery of a patient through the exciter 2 attached to a patient""s arm. The sensor 3 detects oscillations transmitted through the artery and converts them into an electric signal. The phase detector 4 compares the relative phase between the output of the sensor 3 and a reference signal that is the output of the oscillator 1 to produce an in-phase component signal (also referred to as an I-signal below) and a quadrature component signal (also referred to as a Q-signal below). The I-signal and Q-signal outputted from the phase detector 4 are converted by the A/D converters 5 into digital signals and inputted to a phase angle determining circuit 7 and an arc center determining circuit 6. The arc center determining circuit 6 determines the center of distribution of inputs and outputs x- and y-coordinates thereof to the phase angle determining circuit 7. The phase angle determining circuit 7 calculates phase angles of the I-signal and the Q-signal as viewed from the center of the distribution. A blood pressure measuring 8 actuates a cuff 9 at given time intervals and outputs systolic and diastolic blood pressure values to a blood pressure determining circuit 10. The blood pressure determining circuit 10 determines a relation among the systolic and diastolic pressure values and the phase angle determined by the phase angle determining circuit 7 at the instant when the blood pressure measuring device 8 starts to operate, which will be referred to as calibration below. After such calibration, the blood pressure determining circuit 10 determines the blood pressure only based on the phase angle determined by the phase angle determining circuit 7 and outputs it in the form of a continuous wave signal to the display 11. The control circuit 12 are connected to the above described system components to control operations thereof.
If the I-signals and the Q-signals digitized by the phase detector 4 are defined as representing x-coordinates and y-coordinates on a two-dimensional plane, respectively, then they are distributed, as shown in FIG. 9, along an arc. Specifically, the signals outputted from the A/D converters 5 may be used as coordinate data indicating x- and y-coordinates of a sample data (i.e., the detected oscillation). A vector from the origin to each plotted sample point represents the phase and amplitude of a waveform of the oscillation transmitted through the arm of the patient and may be separated into two components: one is an oscillation component transmitted through the blood vessel, that is, a vector from the arc center C, as shown in FIG. 9, to each sample P and the other is an oscillation component transmitted through tissue other then the blood vessel, that is, a vector from the origin O to the arc center C.
Since the tissue other then the blood vessel are not moved, the oscillation component transmitted through the tissue is fixed or stays at the arc center C, while the oscillation component transmitted through the blood vessel has the phase which increases when the blood pressure is high because the elasticity of the blood vessel is sensitive to the blood pressure so that it increases with an increase in blood pressure, thus resulting in quick transmission of the oscillation and decreases when the blood pressure is low because the elasticity of the blood vessel decreases with a decrease in blood pressure, thus resulting in slow transmission of the oscillation. Specifically, the samples move, in time sequence, from one end of the arc to and return from the other end of the arc in each cycle of change in blood pressure or one heartbeat. The phase angle of the sample P, as viewed from the arc center C, has a one-to-one correspondence to the blood pressure. It is, thus, possible to have the systolic pressure Psys and the diastolic pressure Pdias measured by the blood pressure determining circuit 8 bear a one-to-one correspondence to the maximum value xcfx86 sys and the minimum value xcfx86 dias of the phase angle immediately before or after actuation of the cuff 9, respectively. Assuming that the pressure difference between Psys and Pdias is proportional to the phase angle difference between xcfx86 sys and xcfx86 dias, we obtain                     P        =                                                            Psys                -                Pdias                                                              φ                  ⁢                                      xe2x80x83                                    ⁢                  sys                                -                                  φ                  ⁢                                      xe2x80x83                                    ⁢                  dias                                                      ⁢                          (                              φ                -                                  φ                  ⁢                                      xe2x80x83                                    ⁢                  dias                                            )                                +          Pdia                                    (        1        )            
where P is the blood pressure at an phase angle of xcfx86.
It is important for precise measurement of the blood pressure in the above system to determine the arc center C of a distribution of samples with high accuracy, but U.S. Pat. No. 5,590,649 is silent about the algorithm for determining the arc center C. The arc center C may be determined by defining a circle using three samples on the arc and calculate the center of the circle mathematically. The samples derived at regular time intervals usually concentrate in an area d, as shown in FIG. 9. Thus, if the number of samples is decreased to 1/n, and the circle is defined by three selected samples adjacent in time each other, a disadvantage is encountered that a point Cxe2x80x2, as shown in FIG. 9, closer to the arc is determined as the center C due to noise components in the area d. Further, if the selected samples are too close to each other, a segment defined thereby may be a straight line, thus resulting in difficulty in determining the center of the distribution of the samples accurately.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a blood pressure measuring system designed to determine the above described center of an arc needed to measure the blood pressure with high accuracy.
According to one aspect of the invention, there is provided a physical parameter measuring system comprising: (a) an exciter applying oscillations to a subject; (b) a sensor monitoring the oscillations propagated through the subject to provide a signal indicative thereof in sequence; (c) a phase detector detecting a phase of the signal outputted from the sensor to map, in sequence, the signal on a two-dimensional plane as a sample point and providing a sample point signal indicative thereof; (d) an A/D converter converting the sample point signal into a digital sample point signal in sequence; (e) an arc center determining circuit determining a center of an arc defined by a distribution of the sample points mapped for a given period of time, the arc center determining circuit including a sample group selecting circuit selecting the sample points which are located at an interval greater than a given reference sample-to-sample distance to form a sample group consisting of at least three of the selected sample points, and a circle center determining circuit determining a center of a circle passing through the sample group as the center of the arc; and (f) a phase angle determining circuit determining a phase angle of the sample point as viewed from the arc center determined by the arc center determining circuit and providing it as a parameter used to determine a physical parameter of the subject.
In the preferred mode of the invention, the arc center determining circuit also includes a reference distance determining circuit determining the reference sample-to-sample distance according to a given algorithm.
The reference distance determining circuit determines the reference sample-to-sample distance based on the center of the distribution of the sample points.
The sample group selecting circuit forms a plurality of sample groups each consisting of at least three of the selected sample points which are located at the interval away from each other. The circle determining circuit determines centers of circles passing through the sample groups and finds a middle of the centers as the center of the arc.
The subject is a living body. The physical parameter of the subject is a physiological parameter produced by heartbeat motion of the living body.
The reference distance determining circuit includes a center determining circuit determining a center of the distribution of the sample points derived for a time period more than or equal to one cycle of heartbeats, a maximum value determining circuit determining a maximum value of distances between the center of the distribution and the sample points derived for a time period more than or equal to the one cycle of heartbeats, respectively, and a monotone increasing function circuit determining as the reference sample-to-sample distance a value of a monotone increasing function taking the maximum value as a variable.
The maximum value determining circuit may determine as the maximum value the nth (n is two or more) greatest of the distances between the center of the distribution and the sample points.
The monotone increasing function is a proportional function.
A constant of proportion in the proportional function ranges from ⅓ to 2.
A low-pass filter may also be provided which filters the center of the circle passing through the sample group.
According to another aspect of the invention, there is provided a physical parameter measuring method comprising the steps of: (a) applying oscillations to a subject; (b) monitoring the oscillations propagated through the subject to provide a signal indicative thereof in sequence; (c) detecting a phase of the signal to map, in sequence, the signal on a two-dimensional plane as a sample point and providing a sample point signal indicative thereof; (d) converting the sample point signal into a digital sample point signal in sequence; (e) determining a center of an arc defined by a distribution of the sample points mapped for a given period of time, the arc determining step including selecting the sample points which are located at an interval greater than a given reference sample-to-sample distance to form a sample group consisting of at least three of the selected sample points, and determining a center of a circle passing through the sample group as the center of the arc; and (f) determining a phase angle of the sample point as viewed from the arc center determined by the arc center determining circuit and providing it as a parameter used to determine a physical parameter of the subject.
The arc center determining step also includes a reference distance determining step of determining the reference sample-to-sample distance according to a given algorithm.
The reference distance determining step determines the reference sample-to-sample distance based on the center of the distribution of the sample points.
The sample group selecting step forms a plurality of sample groups each consisting of at least three of the selected sample points which are located at the interval away from each other, and wherein the circle determining step determines centers of circles passing through the sample groups and finds a middle of the centers as the center of the arc.
The subject is a living body, and the physical parameter of the subject is a physiological parameter produced by heartbeat motion of the living body.
The reference distance determining step includes a center determining step of determining a center of the distribution of the sample points derived for a time period more than or equal to one cycle of heartbeats, a maximum value determining step of determining a maximum value of distances between the center of the distribution and the sample points derived for a time period more than or equal to the one cycle of heartbeats, respectively, and a monotone increasing function step of determining as the reference sample-to-sample distance a value of a monotone increasing function taking the maximum value as a variable.
The maximum value determining step determines as the maximum value the nth (n is two or more) greatest of the distances between the center of the distribution and the sample points.
The monotone increasing function is a proportional function.
A constant of proportion in the proportional function ranges from ⅓ to 2.
A low-pass filtering step is further provided which filters the center of the circle passing through the sample group.